The present invention relates to a laser machining method and a laser machine, and particularly relates to a laser machining method and a laser machine suitable for machining a printed board.
When a blind hole (hereinafter simply referred to as xe2x80x9cholexe2x80x9d) for making a connection between layers is machined by a laser beam in a built-up type printed board, a conformal mask method or a direct method is adopted. In the case of the conformal mask method, an insulating layer is irradiated with a laser beam through an etching window which is formed by removing an outer-layer copper foil by etching in advance. On the other hand, in the case of the direct method, an insulating layer having no outer-layer copper foil is irradiated with a laser beam directly. Thus, the insulating layer formed of resin containing glass reinforced fiber or filler is removed by the laser energy. In some laser machines, a laser beam outputted from a laser oscillator is supplied to a plurality of machining heads so that the machining speed is enhanced. Such a laser machine will be described with reference to FIG. 10.
FIG. 10 is a configuration view of a background-art laser machine. A laser oscillator 1 outputs a pulsed laser beam 2. A half mirror 3 transmits about 50% of the laser beam 2 incident thereto and reflects the rest of the laser beam 2. Hereinafter, the laser beam 2 transmitted through the half mirror 3 will be referred to as a transmitted beam 2a, and the laser beam 2 reflected on the half mirror 3 will be referred to as a reflected beam 2b. The reflecting surfaces of total reflection corner mirrors 4a to 4c are fixed. As indicated by the arrows in FIG. 10, galvanomirrors 5a to 5d rotate desirably around the rotation axes thereof so that the reflecting surfaces thereof can be positioned at desired angles respectively. Condensing lenses (fxcex8 lenses) 6a and 6b are held by a first machining head 7a and a second machining head 7b respectively. A printed board 8 is fixed to an X-Y table 9. A scan area 10a of the galvanomirrors 5a and 5b and a scan area 10b of the galvanomirrors 5c and 5d measure about 50 mm by 50 mm respectively.
Next, the operation of the background-art laser machine will be described.
The laser beam 2 outputted from the laser oscillator 1 is split into the transmitted beam 2a and the reflected beam 2b by the half mirror 3. The transmitted beam 2a is reflected by the total reflection corner mirrors 4a and 4b to be made incident onto the galvanomirror 5a, passed through an optical path defined by the galvanomirrors 5a and 5b, and condensed by the condensing lens 6a so as to machine a hole in the scan area 10a. The reflected beam 2b is reflected by the total reflection corner mirrors 4c to be made incident onto the galvanomirror 5c, passed through an optical path defined by the galvanomirrors 5c and 5d, and condensed by the condensing lens 6b so as to machine a hole in the scan area 10b. Then, the galvanomirrors 5a to 5d are operated so that the machining head 7a machines the hole in the scan area 10a and the machining head 7b machines the hole in the scan area 10b, sequentially. After the holes in the scan areas 10a and 10b have been machined down, the X-Y table 9 is moved so that machining in the next scan areas 11a and 11b is performed. Incidentally, a distance L between the machining head 7a and the machining head 7b is designed to be adjustable. The distance L is adjusted in advance so that the scan area 10a and the scan area 10b are not put on each other and the number of times to move the X-Y table 9 is minimized.
Incidentally, in order to machine a hole, a plurality of pulsed laser beams 2 (hereinafter, a pulse of laser beam will be referred to as a xe2x80x9claser pulsexe2x80x9d) are often radiated. A machining method in which a plurality of laser pulses are radiated continuously for one hole and the next hole is machined after the preceding hole has been machined down, is called xe2x80x9cburst machiningxe2x80x9d. A machining method in which a plurality of holes are grouped into one set, every hole in one set is irradiated with one laser pulse, and this operation is repeated till the holes in the one set have been machined down, is called xe2x80x9ccycle machiningxe2x80x9d.
FIG. 11 is a timing chart of respective portions in the cycle machining; (a) designates a start signal for the laser oscillator 1; (b) designates the magnitude of energy of the laser beam 2; (c) designates the magnitude of energy of the transmitted beam 2a; (d) designates a positioning signal for the galvanomirrors 5a and 5b; (e) designates the magnitude of energy of the reflected beam 2b; and (f) designates a positioning signal for the galvanomirrors 5c and 5d. 
When the start signal is turned ON (at time T0), the radiation of the laser beam 2 is started after a delay period TDL of several xcexcs has passed (at time T1, in this case, TDL). The magnitude of the energy increases gradually and reaches substantially a peak value WP after a rising period TR has passed (at time T2). When the start signal is turned OFF after a pulse period TP has passed since the time T0 (at time T3), the energy decreases gradually and reaches 0 after a falling period TD has passed (at T4). Then, the galvanomirrors 5a to 5d are operated during a period TG after the time T5 so as to be positioned in the next machining positions. After the positioning is completed (at time T6), the start signal is turned ON again (at time T7). The above-mentioned operation is repeated hereafter. In this case, since the transmitted beam 2a and the reflected beam 2b are obtained by splitting the laser beam 2, each of the beams 2a and 2b has energy the peak value of which is WP/2. Incidentally, if the time T5 is set to be simultaneous with the time T4, and if the time T7 is set to be simultaneous with the time T6, the machining speed can be accelerated.
The laser pulse period during which the laser oscillator can oscillate is 0.33 ms (frequency: 3 kHz), and the pulse period TP is several tens of xcexcs. On the other hand, the period TG required for positioning the galvanomirrors 5c and 5d is about 2 ms, and the period required for positioning the table is about 200 ms. Therefore, burst machining can accelerate increase the machining speed in comparison with cycle machining.
However, in the case where burst machining is performed by the conformal mask method, if the pulse period is set to be not longer than 2 ms, decomposed flying matters generated by a laser pulse previously radiated remain inside and near the hole. Then, the remaining decomposed flying matters absorb the energy of a succeeding laser pulse so as to be high-temperature plasma. Thus, the resin in the flank of the hole is hollowed so that the diameter of the intermediate portion of the hole in the direction of depth is expanded to be larger than the diameter of the upper or lower portion. Thus, the hole is formed into a so-called barrel-like hole, so that the quality of the hole deteriorates.
Moreover, in the case where burst machining is performed by the direct machining method, if the insulating material is of FR-4 which contains glass reinforced fibers, only the resin is hollowed due to the difference in decomposition energy between the resin and the glass (resin:glass=1:3 to 4). Thus, the glass fibers project over the flank of the hole so that the quality of the hole deteriorates.
Further, even in a laser of RF excitation which rises quickly, the period TR to reach the peak value WP is about 15 xcexcs as shown in FIG. 11. Thus, it is impossible to obtain the peak value WP in a range where the pulse width is not longer than 15 xcexcs.
In addition, since the falling period TD after the start signal is turned OFF is in a range of from 30 xcexcs to 50 xcexcs, the real pulse width becomes longer than the set pulse width TP so that the supplied energy becomes excessive. If excessive energy is supplied, a resin residue remains on the bottom surface of the hole, the surface roughness of the internal wall becomes large, or the internal wall is carbonized. In either case, the quality of the hole deteriorates. In addition, there is a fear that the copper foil of the internal layer is damaged or the resin on the back of the copper foil is peeled off.
Further, in order to obtain N split beams on the assumption that the peak value per head required for machining is WP, the laser oscillator must have a large capacity and the peak value of NWP, that is, N times as large as WP.
In addition, in the case where a laser beam is split by a half mirror, the transmitted beam 2a and the reflected beam 2b are produced simultaneously, so that a time difference cannot be given between the transmitted beam 2a and the reflected beam 2b. Therefore, the number of spots to be machined in the scan area 10a must be the same as that in the scan area 10b. Thus, the kind of the printed board which can be machined is limited. In addition, it is difficult to make the number of heads odd.
It is an object of the present invention to solve the foregoing problems in the background art and to provide a laser machining method and a laser machine in which a laser oscillator is used effectively and machining energy is controlled accurately so that holes which are superior in quality can be machined.
In order to achieve the above object, the present invention is designed such that optical path deflecting means for deflecting an optical path of laser beam is disposed on the optical path of the laser beam, and laser energy supplied to a portion to be machined is controlled by the optical path deflecting means.
In this case, a pulsed laser beam is used, and the laser beam supplied to the aforementioned portion to be machined is formed to have a substantially rectangular waveform. In addition, one pulse laser beam outputted from a laser oscillator is time-divided, and the portion to be machined is irradiated with the time-divided pulsed laser beam.
In addition, the present invention is also designed such that; in a laser machine in which laser beam outputted from a laser oscillator is supplied to a plurality of machining heads; optical path deflecting means for deflecting an optical path of laser beam are provided, the number of the optical path deflecting means being equal to the number of the machining heads; the optical path deflecting means are disposed on the optical path; and pulsed laser beam is supplied to one of the machining heads.
In this case, apparatus of an acousto-optic system is used as each of the optical path deflecting means. In addition, the optical path deflecting means are disposed in series with each other on the optical path of the laser beam. Polygonal mirrors may be used as the optical path deflecting means.